1. Field of the Invention
This invention relates generally to an airfoil for aircraft, and more particularly, to an airfoil that has improved stability and performance characteristics through a wide range of airspeeds.
2. Description of the Prior Art
The present invention constitutes an improvement over applicants' previously patented unique airfoil disclosed in U.S. Pat. No. 3,706,430, dated Dec. 19, 1972, entitled AIRFOIL FOR AIRCRAFT. The airfoil of that patent included a wedge-like section formed by a continuous first surface which extended between the leading and trailing edges, and a second surface that was joined to the first surface along the leading edge. The second surface projected rearwardly in the direction of the trailing edge and terminated materially in advance thereof to define a step-like discontinuity of the airfoil. The thickness of the airfoil gradually increased from the leading edge to approximately 50% of the chord to form the wedge-like section, at which point, the second surface was sharply projected in the direction of the first surface to form the step-like discontinuity.
It was found from flight tests of small airplane models constructed having the above described airfoil, that the tested airfoil demonstrated unusually good lift, stability and pitching moment characteristics. For example, the airfoil greatly resisted stalling in that it was necessary to induce an angle of attack between 30.degree. - 40.degree. before the stall occurred, as distinguished from angle of attack values between 18.degree. - 22.degree. common to conventional airfoils. The technical explanation for such improved performance is unknown to the inventors and appears to be contrary to accepted aerodynamic theory. However, since the effect of stalling plays a significant role in accident reports, it is apparent that by resisting stalling, the above airfoil can materially increase the safety of flight.
Continued tests have shown that the patented airfoil has a poor glide ratio in the range between 3:1 - 4:1. While various lift generating devices, such as flaps, are known to be employed on conventional airfoils to increase the glide ratio such flaps are commonly mounted at the trailing edge of the airfoil. However, the use of such flaps on applicants' airfoil is not practical due to the relative thinness of said airfoil at its trailing edge. Furthermore, the portion of the airfoil downstream of the step is not intended to be used for housing auxiliary equipment, such as flaps, that would tend to eliminate or markedly reduce the aerodynamic effect of the step-like discontinuity.
During the course of further tests, applicants decided to place a lift generating member, such as a flap, adjacent the step-like discontinuity of the airfoil. As a general matter, the placement of a flap on either the top or lower surface of an airfoil is well-known wherein such flaps serve as spoilers to decrease the lift and increase the drag. However, contrary to applicants' expectation, the placement of a flap adjacent the step-like discontinuity of the airfoil produced increased lift when the flap was moved to an extended or deflected position in the airstream. Thus, applicants have improved their previously patented airfoil by incorporating therein lift generating means to increase the glide ratio thereof.
As used herein, the term airfoil is defined as a body, such as an airplane wing, designed to provide a desired reaction force when in motion relative to the surrounding air.
Applicants' improved airfoil is to be distinguished from a class of airfoils designated as being "supercritical." In this regard, all airfoils have a characteristic known as critical Mach number (Mcr) which is the air speed ratioed to the speed of sound (Mach 1) at which the flow over some portion of the airfoil just reaches Mach 1. Airfoils usually are designed to fly below their critical Mach number because of the high drag rise caused by the formation of shock waves and, possibly, flow separation associated with super-critical speeds. However, the supercritical airfoil is capable of flying close to the speed of sound (Mach 1) without experiencing the high drag rise associated with more conventional shapes.
The conventional airfoil, particularly one designed to operate in the subsonic speed range, has cambered surfaces that define a profile of gradually decreasing thickness from the leading edge to the trailing edge. The supercritical airfoil has a much flatter shape of the upper surface thereof which reduces both the extent and strength of the shock wave, as well as the adverse pressure rise behind the shock wave, with corresponding reductions in drag. To compensate for the reduced lift of the upper surface of the supercritical airfoil resulting from the reduced curvature, the airfoil has increased camber near the trailing edge. This is to be distinguished from applicants' unique airfoil which has substantially no camber at the trailing edge. Furthermore, the supercritical airfoil does not provide for any step-like discontinuity as incorporated in applicants' airfoil.
Applicants unique airfoil is also to be distinguished from those incorporating leading edge extensions and fences, such as shown and disclosed in U.S. Pat. No. 2,802,630, dated Aug. 13, 1957 entitled WING LEADING EDGE DEVICE. The use of such extensions apparently has value in the design of sweptback wings in obtaining a more perfect airflow over the outboard portion of the wing. Applicants' improved airfoil, as will become more apparent hereinafter, is directed toward the use of lift generating members located in the region of the step-like discontinuity to improve the aerodynamic characteristics thereof.